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    SubjectRe: [PATCH v7 24/27] x86: Add swiotlb force off support
    On Fri, Dec 21, 2012 at 07:23:03PM -0800, Eric W. Biederman wrote:
    > Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> writes:
    >
    > > On Fri, Dec 21, 2012 at 06:42:47PM -0800, Eric W. Biederman wrote:
    > >> Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> writes:
    > >>
    > >> > On Mon, Dec 17, 2012 at 11:15:56PM -0800, Yinghai Lu wrote:
    > >> >> So use could disable swiotlb from command line, even swiotlb support
    > >> >> is compiled in. Just like we have intel_iommu=on and intel_iommu=off.
    > >> >
    > >> > You really need to spell out why this is useful.
    > >>
    > >> YH why can't we safely autodetect that the swiotlb is unusable when
    > >> there is no memory below 4G free?
    > >
    > > I am not sure what 'YH' stands for (Yeah?).
    >
    > Yinghai Lu's nickname.
    >
    > > However we could turn SWIOTLB off altogether if it cannot allocate
    > > _some_ memory. It could try first 64MB, then 32MB, lastly 16MB. And
    > > if all that fails - print a nice warning and continue on.
    > >
    > > Later in the late initialization phase, when pci_swiotlb_late_init
    > > is called - it can then figure out whether 'iommu' has been set
    > > and it iself was never able to allocate. At that point it can try
    > > the dynamic allocation (swiotlb_late_init_with_default_size)
    > > ... and if that fails give up and panic.
    >
    > As far as I can tell panics should be avoided unless there is something
    > that actually needs an iommu, and the swiommu is the only option, and
    > the swiommu can not fulfill that request.

    Right.
    >
    > In this case YH has been working on the case of loading a kernel
    > completely above 4G, and apparently he has also been testing the case of
    > running a kernel with no memory below 4G.
    >
    > Eric
    >


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